1. Field of the Invention
The present invention relates to a photovoltaic device and a process for producing the same. More particularly, the invention relates to a photovoltaic device having a pin junction and a production process therefor.
2. Description of Related Art
In the history of development of thin film solar cells having pin junctions, various attempts have been made with regard to doped layers on a photoreceptive side since these layers are important factors for improving conversion efficiency (.eta.).
Particularly, a variety of studies are being made about a p-layer, which is one of the doped layers on the photoreceptive side and functions as an hydrogenated amorphous silicon window layer. Since the p-layer is not a photoelectric conversion layer, the p-layer must possess a characteristic of absorbing only a small amount of light. At the same time, the p-layer must also have a high electrical conductivity and good i/p interface characteristics. A variety of studies are being made in order to achieve a p-layer which can satisfy these contradictory characteristics.
For example, Japanese Examined Patent Publications Nos. HEI 3(1991)-40515 and HEI 3(1991)-63229 disclose the use of an a-SiC:H (hydrogenated amorphous silicon carbide) film doped with boron atoms as the p-layer. In these publications, the p-layer is formed by decomposing B.sub.2 H.sub.6 gas together with a mixture gas of silane or a silane derivative (e.g., SiH.sub.4), a hydrocarbon (e.g., CH.sub.4), an inert gas (e.g., Ar, He) and the like by glow discharge. Other generally known methods of forming the p-layer include a plasma chemical vapor deposition (plasma CVD).
However, where B.sub.2 H.sub.6 gas is mixed with gas sources, boron atoms extract hydrogen atoms terminating bonds of silicon atoms, etc., in the amorphous layer. That gives rise to a large number of unbonded bonds, which are referred to as dangling bonds. For this reason, if the boron-doped amorphous film formed by the above-mentioned method is used as a window p-layer for a solar cell, the light absorption in the p-layer may rise.
In order to suppress a rise in the light absorption, carbon atoms are introduced in the layer up to several ten percents. However, an increase in the number of carbon atoms may bring about deterioration of the quality of the layer. As a result, the electrical conductivity declines and the inner resistance of the entire device increases.
Thus a technical problem exists in that to provide a desired conductivity which does not give rise to series resistance for cell characteristics will lead to a non-negligible increase in the light absorption and therefore a sufficient photoelectric current will not be ensured.
By the plasma CVD, boron atoms in plasma increase dangling bonds also on the surface of the film, and thereby gives rise to a large number of recombination levels at a p-i interface, As a result, the conversion efficiency is affected adversely.
For example, in the case where a boron-doped SiC film is used as a p-layer, the film has a poor connection with the photoelectric conversion layer and becomes a recombination center of generated photo-carriers. Therefore a sufficient open-circuit voltage (Voc) and a satisfactory fill factor (F.F.) cannot be ensured.
To cope with these drawbacks, it is generally known to place an amorphous film in which the amount of carbon atoms are gradually changed or an intrinsic SiC:H film, as a buffer layer at the p-i interface for reducing adverse effects on solar cell characteristics.
However, such buffer layers are poor in electrical conductivity and can cause an increase in the inner resistance of the device. Eventually, it is impossible to avoid a decline in the F.F.
On the other hand, as regards the formation of the player, Japanese Unexamined Patent Publication HEI 7(1995)22638 proposes a process of forming a hydrogenated amorphous p-type silicon layer by forming a hydrogenated amorphous boron (a-B:H) layer and subsequently disposing an hydrogenated amorphous silicon layer thereon, and Appl. Phys. 36 (1997) 467 proposes a process of forming a p-layer by forming a hydrogenated amorphous boron layer and subsequently forming an amorphous carbon layer (a-C) thereon.
However, it is still difficult to reduce the light absorption in the hydrogenated amorphous boron layer sufficiently.
In usual cases, used as a substrate for forming photovoltaic devices is a glass substrate having thereon an electrically conductive transparent oxide film, for example, of SnO.sub.2 or ZnO with projections and depressions. In the case where the pin junction is formed on such an electrically conductive transparent oxide film, resistance increases at the interface between the electrically conductive transparent oxide film and the hydrogenated amorphous boron layers proposed by Japanese Unexamined Patent Publication HEI 7(1995)-22638 and Appl. Phys. 36 (1997) 467. Therefore, it is still difficult to obtain good cell characteristics.
So-called tandem-type photovoltaic devices are also generally known. The tandem-type photovoltaic devices are constructed to have a plurality of layered pin junctions in order to utilize spectra of light of the sun efficiently to improve a photoelectric conversion rate. In such photovoltaic devices, by optionally setting optical band gaps in photoelectric conversion layers of pin junctions, photocurrent generated in each pin junction can be efficiently utilized. In other words, in the tandem-type photovoltaic device, which is a device having two or three or more of such pin junctions connected to each other, an interlayer p-layer which is adjacent to an n-layer is usually formed as a laminate of a contact layer of a highly doped a-Si:H film for obtaining an ohmic contact with the underlying adjacent n-layer and a wide gap silicon alloy film, e.g., an a-SiC:H film, an a-SiO:H film, etc.
However, interfaces inevitably exist between p-layers and adjacent n-layers in intermediate layers. Accordingly, presently, the quality of films must be sacrificed for obtaining good ohmic contact, and recombination layers of about 3 nm thickness which have a large light absorption must be inserted at the interfaces. Moreover, there is a problem that the F.F. of the photovoltaic device declines since the light absorption by the contact layer which is not an active layer brings about a large loss and since the series resistance of the gap-widened silicon alloy film of a-SiC:H, a-SiO:H or the like becomes too large to neglect.
Thus, none of the above-described conventional methods has not achieved a technique which satisfies the contradictory characteristics required for the p-layer and the tandem-type photovoltaic device having a plurality of laminated pin junctions. The contradictory characteristics are a low light absorption, a high electrical conductivity and good interface characteristics both to the electrically conductive transparent oxide film or the n-type hydrogenated amorphous silicon layer (or a microcrystalline silicon layer) and to the photoelectric conversion layer.